Antenna beam

ABSTRACT

A fixed phase shift for each of a plurality of radio frequency signal components directed to or received from a plurality of antenna elements is formed in a phase shifter. A desired antenna beam pattern with at least one grating lobe is formed on the basis of the phase-shifted radio frequency signal components of the antenna elements in a predefined antenna structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Phase application of InternationalApplication No. PCT/FI2008/050569, filed Oct. 13, 2008, which isincorporated herein by reference in its entirety.

BACKGROUND

1. Field

The invention relates to an antenna beam of an antenna having aplurality of elements.

2. Description of the Related Art

A Butler matrix with an antenna array designed for the Butler matrix canbe used to form a discrete set of orthogonal beams. Each beam isdirected towards a predetermined direction which depends on the phaseshifts generated in the Butler matrix. A typical number of beams is fourto eight.

In a Butler matrix, phase shifts of different signals are typicallyformed in passive analog circuits, and hence the phase shifts of signalscannot be changed without changing the whole a Butler matrix. Thestructure of an antenna array utilizing a Butler matrix is such that thespacing between two successive elements in the antenna array is roughlyhalf a wavelength of the radio frequency radiation in order to formdesirable lobes and avoid grating lobes.

However, there are problems involved with the design. The present use ofa Butler matrix and an antenna array leaves no room for changing theantenna beam pattern and the configuration cannot flexibly be applied,for example, to omni-directional sites or to a site needing certaindirectionality.

There have been attempts to solve the problems. Digital beam forming atbase band helps with the lobes but adds drastically to the complexity oftransmitters and receivers. Furthermore, digital beam forming requiresaccurate on-line calibration and brings along a lot of additionalcomplexity to the base band processing such as estimation of spatialradio channel characteristics. Hence, there is a need for simpler andstill flexible antenna configurations.

SUMMARY

An object of the invention is to improve beam forming. According to anaspect of the invention, there is provided a method of forming anantenna beam pattern in a radio system. The method further comprisesforming a fixed phase shift for each of a plurality of radio frequencysignal components directed to or received from the plurality of antennaelements; and forming a desired antenna beam pattern with at least onegrating lobe on the basis of the phase-shifted radio frequency signalcomponents of the antenna elements in a predefined antenna structure.

According to another aspect of the invention, there is provided anantenna. The antenna comprises a phase shifter; a plurality of elements;the phase shifter being configured to form a fixed phase shift for eachof a plurality of radio frequency signal components directed to orreceived from the plurality of antenna elements; and the antennaelements having a structure configured to form a desired antenna beampattern with at least one grating lobe using the fixed phase shifts onthe basis of the phase-shifted radio frequency signal components of theantenna elements.

According to another aspect of the invention, there is provided atransmitter. The transmitter comprises an antenna which comprises aphase shifter; a plurality of elements; the phase shifter beingconfigured to form a fixed phase shift for each of a plurality of radiofrequency signal components directed to the plurality of antennaelements; and the antenna elements having an antenna structureconfigured to form a desired antenna beam pattern with at least onegrating lobe using the fixed phase shifts on the basis of thephase-shifted radio frequency signal components of the antenna elements.

According to another aspect of the invention, there is provided areceiver. The receiver comprises an antenna which comprises a phaseshifter; a plurality of elements; the phase shifter being configured toform a fixed phase shift for each of a plurality of radio frequencysignal components received from the plurality of antenna elements; andthe antenna elements having an antenna structure configured to form adesired antenna beam pattern with at least one grating lobe using thefixed phase shifts on the basis of the phase-shifted radio frequencysignal components of different antenna elements.

The invention provides several advantages. A desired antenna pattern canbe formed without calibration and complexity.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail withreference to embodiments and the accompanying drawings, in which

FIG. 1 shows a transmitter;

FIG. 2 illustrates a receiver;

FIG. 3 illustrates different beams;

FIG. 4 illustrates an omni-directional antenna beam pattern;

FIG. 5 illustrates an antenna pattern radiating to an angle of 120°;

FIG. 6 illustrates different beams transmitting to different receivers;

FIG. 7 illustrates beams of different data directed to a receiver;

FIG. 8 illustrates beams of different data of common encoding directedto a receiver;

FIG. 9 illustrates switching between different modes; and

FIG. 10 presents a flow chart of the method.

DETAILED DESCRIPTION

The following embodiments may be applied to radio frequency signals invarious radio systems. The applications include, for example, WiMAX(Worldwide Interoperability for Microwave Access), HSPA (High-SpeedPacket Access), 3GPP (Third Generation Partnership Project) LTE (LongTerm Evolution). It may also be applied with different physical layermultiple access techniques such as CDMA (Code Division Multiple Access),WCDMA (Wide-band CDMA), FDMA (Frequency Division Multiple Access), TDMA(Time Division Multiple Access), OFDMA (Orthogonal Frequency DivisionMultiple Access). It also applies to TDD (Time Division Duplex) and FDD(Frequency Division Duplex.

FIG. 1 presents a transmitter comprising an antenna array 100, a phaseshifter 102, radio frequency module 104, and a base band module 106. Thebase band module 106, which may comprise a processor, memory and asuitable computer program, may perform base band signal processing andfeed the signal through an optical connection to the radio frequencymodule 104. The base band module 106 may instead or additionallycomprise programmable hardware, such as FPGA (Field Programmable GateArray). The optical connection may be an optical fiber, for example.Instead of an optical connection, a wired electrical or a wirelessconnection may be used. The radio frequency module 104 may mix a baseband signal to a radio frequency signal and feed the radio frequencysignal to the phase shifter 102. The phase shifter 102 may be an analogphase shifting circuit which may be an analog beam forming component,such as a Butler matrix, microstrips, stripline circuit or the like. Thephase shifter 102 divides the signal to be transmitted into at least twosignal components and each signal component is phase-shifted by apredetermined amount. Each signal component is fed to an element of theantenna array 100. A desired transmission antenna beam pattern is formedon the basis of interference of the phase-shifted radio frequency signalcomponents transmitted through different elements 100A, 100B of theantenna 100. Two antenna elements 100A, 100B are shown in FIG. 1 but ingeneral there may be more than two elements. The implementation of thephase shifter may affect the required signaling throughout the system.For example, the number of signal paths from a base band module to thephase shifter may change according to the phase shifter used.

FIG. 2 presents a receiver comprising an antenna array 200, a phaseshifter 202, a radio frequency module 204 and a base band module 206which may comprise a processor, memory and a suitable computer program.The antenna array 200 having elements 200A, 200B receives anelectromagnetic signal at radio frequency and each element 200A, 200Bfeeds the signal components to the phase shifter 202. The phase shifter202 phase-shifts each radio frequency signal component to apredetermined extent and combines the signal components. This leads toselective reception with respect to the direction of the signals i.e.reception beams. The combined signal is then mixed to base band in theradio frequency module 204 and the base band signal is fed to the baseband module 206 for signal processing. A desired reception antenna beampattern may be formed on the basis of interference of the phase-shiftedradio frequency signal components received from the different elements200A, 200B of the antenna 200. Two antenna elements 200A, 200B are shownin FIG. 2 but in general there may be more than two elements.

In an embodiment, the radio frequency module 104 and the phase shifter102 may be integrated together such that they form a unified structure.Similarly, the radio frequency module 204 and the phase shifter 202 maybe integrated together.

In an embodiment, the phase shifter 102 and the antenna 100 may beintegrated together such that they form a unified structure. Similarly,the phase shifter 202 and the antenna 200 may be integrated together.

In an embodiment, the radio frequency module 104, the phase shifter 102,and the antenna 100 may be integrated together such that they form aunified structure. Similarly, the radio frequency module 204, the phaseshifter 202, and the antenna 200 may be integrated together.

In the phase shifter 102, 202, each fixed phase shift may actually bedesigned for an antenna array having elements in such a predefinedstructure that a discrete set of orthogonal beams are formed. Thepredefined structure usually requires that the distance betweensuccessive antenna elements should be less than a wavelength of theradio frequency carrier.

In antenna arrays having a distance between successive elements greaterthan one half of a wavelength of a carrier frequency special side lobesdue to an aliasing effect are formed. The aliasing effect results fromthe fact that the Nyqvist sampling criterion in spatial domain is notfulfilled. As can be seen in FIG. 3, the special side lobes (sidemaximums) called grating lobes 302, 304 have a substantially largerpower than the usual side lobes 306, and may have at least nearly thesame power level as the main lobe 300. The horizontal axis denotes theazimuth angle in an arbitrary scale and the vertical axis is themagnitude of a beam pattern. If the Nyqvist sampling criterion isfulfilled, grating lobes can be avoided at all scan angles. However, fora limited angular aperture (scan angles) the grating lobes can beavoided even if the inter-element spacing is somewhat larger than onehalf of a carrier wavelength. For example, in the case of linear arrays,the limiting value (condition for grating lobes) for the element spacingmay be mathematically expressed as follows:

d/λ<1/(1+sin|θ_(max)|)  (1)

where d is the distance between successive elements, λ is the wavelengthof a carrier frequency, and θ_(max) is the maximum angle between a mainbeam of the antenna pattern and an axis of the array (i.e. so calledmaximum scan angle). If the element spacing is half a wavelength orless, grating lobes are avoided at all scan angles. If the elementspacing is between half a wavelength and one wavelength, maximum scanangle is limited according to equation (1). If the element spacing islarger than a wavelength, grating lobes exist at all scan angles.

However, in an embodiment the structure of the antenna elements isdifferent from a conventional structure providing a discrete set oforthogonal beams without grating lobes. In an embodiment, the distancebetween the antenna elements is set larger than one half of thewavelength of the radio frequency signal in order to create at least onegrating lobe in a desired direction.

In a transmitter or a receiver, a desired antenna beam pattern may beformed with an antenna 100, 200 having a larger spacing between antennaelements 100A, 100B, 200A, 200B than a limiting value given by, forexample, equation (1).

In antenna arrays having a distance between successive elements greaterthan half a wavelength of a carrier, special side lobes due to thealiasing effect are formed. As can be seen in FIG. 3, the special sidelobes called grating lobes 302, 304 have a substantially larger powerthan the usual side lobes 306 and may have at least nearly the samepower level as the main lobe 300. The horizontal axis is the azimuthangle in an arbitrary scale and the vertical axis is the magnitude of abeam. Grating lobes are usually considered highly undesirable. However,the present solution utilizes the grating lobes.

In an embodiment, the spacing of the antenna elements 100A, 100B, 200A,200B may be, for example, about 0.75, 1.25, 1.75 wavelengths of thecarrier frequency of the radio frequency signal. In general, the spacingof the antenna elements may be approximately (x+n*0.5)λ, where 0<x≦0.5,n is a non-negative integer, and λ is a wavelength of the radiofrequency carrier. In this way, grating lobes may be formed deliberatelyfor creating an omni-directional beam pattern. Since the grating lobesare utilized, the spacing of the antenna elements is larger than λ/2,and x and n can be defined so that the desired number and directions ofgrating lobes are obtained. FIG. 4 presents antenna beam patterns 400,402 with a Butler matrix and antenna spacing of half a wavelength andantenna beam patterns 404, 406 with the same Butler matrix and antennaspacing of 1.25 wavelengths for an omni-directional purpose. Thehorizontal axis is the azimuth angle in degrees and the vertical axis isthe magnitude of a beam pattern in an arbitrary scale. As there are onlytwo antenna elements in this example, two orthogonal beams may beformed. It can be noticed that the antenna beam patterns 404, 406 coverthe azimuth directions −90° to −60° and 60° to 90° with a higher gainthan the conventional antenna beam patterns 400, 402. Moreover, theangular space can be divided into two beams, which gives a 3 dB gain inthe beam forming mode.

In an embodiment, the spacing of the antenna elements 100A, 100B, 200A,200B may be, for example, about 1.0, 1.5, 2.0 wavelengths of the carrierfrequency of the radio frequency signal. In general, the spacing of theantenna elements may be approximately (0.5+n*0.5)λ, where n is anon-negative integer, and λ is a wavelength of the radio frequencycarrier. In this way grating lobes may be formed deliberately for havingan antenna radiating to a sector covering an angle of 120°. Since thegrating lobes are utilized, the spacing of the antenna elements islarger than λ/2. FIG. 5 presents antenna beam patterns 500, 502 with aButler matrix and antenna spacing of half a wavelength and antenna beampatterns 504, 506 with the same Butler matrix and antenna spacing of 1.0wavelength for an antenna pattern radiating to a sector covering anangle of 120°. The horizontal axis is the azimuth angle in degrees andthe vertical axis is the magnitude of a beam pattern in an arbitraryscale. As there are only two antenna elements in this example, twoorthogonal beams are formed (one drawn in a solid line and the otherdrawn in a dashed line). It can be noticed that the antenna beampatterns 500, 502 provide less correlation due to orthogonality. Forexample, in the directions of beam pattern maxima of one beam the otherbeam has nulls. Moreover, the angular space can be divided into twobeams which give a 3 dB gain in the beam forming mode.

A desired beam may be formed using the following steps. A desired widthof a sector for the at least one antenna beam pattern may be determined.A number of grating lobes for the sector may be determined. A spacing ofthe antenna elements may be determined. The determination may mean ananalytical calculus, a computer simulation, a measurement of antennapatterns, or recognition of the requirements due to radio networktopology and planning and the allowed antenna size. Phase shifts fordifferent antenna elements may be set using a Butler matrix or a similarphase shift network. The phase angle shifts of the Butler matrix lead toorthogonal beams but other phase shifts result in non-orthogonal beamswhich can also be employed in an embodiment. All of these steps may beperformed in a processor with a suitable computer program or with asuitable electronic circuit, for example in a base band module 106, 206.

In an embodiment of FIG. 6, different beams 604, 606 may be transmittedfrom a transmitter 610 to different receivers 600, 602. The processorwith a suitable computer program in the base band module 106 may controlthat proper data is fed to each beam. Often that means that differentdata is fed to different beams. This embodiment can be considered as abeam forming mode. This is often considered as a MISO (Multiple InputSingle Output) mode of transmission with multiple transmission antennaelements and a single reception antenna element.

In an embodiment of FIG. 7, different data may be transmitted from atransmitter 710 to a receiver 700 through different beams 702, 704. Inthis example, the receiver 700 includes at least two antenna elements.The transmitter 710 may be a base station and the receiver 700 may be asubscriber terminal. The transmission may be performed simultaneously orat different moments. The processor with a suitable computer program inthe base band module 106 may control that the feeding of the data iscarried out to different beams. Correspondingly, different data streamsfrom different antennas of the transmitter 710 may be received throughboth different beams in a receiver 700. Then the base band module 206 ofa receiver may control the reception from different beams using forexample MIMO (Multiple Input Multiple Output) receiver techniques. Thisembodiment can be considered as a spatial multiplexing mode.

In an uplink direction, the transmission link from a subscriber terminalto a base station operates typically in a SIMO (Single Input MultipleOutput) mode in which the signal is transmitted from a single terminalantenna element to a plurality of base station antennas or beams. Thepresent solution can also be applied to the SIMO case.

In an embodiment of FIG. 8, data may be coded and different parts of thecoded data may be transmitted from a transmitter 810, such as a basestation, through different beams 802, 804 to a receiver 800. Theprocessor with a suitable computer program in the base band module 106may perform encoding and control division of the encoded data and thefeeding of parts P1, P2 of the data to the different beams 802, 804.Correspondingly, encoded data may be received from different beams in areceiver. When the encoded data from each beam has been received andcombined the encoded data may be decoded. This embodiment can beconsidered as a transmit diversity technique mode on the basis ofspace-time coding.

In an embodiment of FIG. 9, the transmitter may switch between the beamforming mode and the spatial multiplexing mode, the beam forming modeand the diversity technique mode or the spatial multiplexing mode andthe diversity technique mode. In the base band module 106, the processor900 with a suitable computer program may control the mode switch 902switching between different modes of transmission.

FIG. 10 presents a flow chart of the method. In step 1000, a fixed phaseshift is formed for each of a plurality of radio frequency signalcomponents directed to or received from the plurality of antennaelements. In step 1002, a desired antenna beam pattern with at least onegrating lobe is formed on the basis of the phase-shifted radio frequencysignal components of the antenna elements in a predefined antennastructure. The antenna structure may be configured for orthogonal ornon-orthogonal beams with the fixed phase shifts.

The embodiments may be implemented, for instance, with integratedcircuits, phase shifters, ASIC or VLSI circuits (Application SpecificIntegrated Circuit, Very Large Scale Integration). Alternatively oradditionally, the embodiments of the method steps may be implemented asa computer program which may be produced and distributed as a product.

The computer program may be stored on a computer program distributionmedium readable by a computer or a processor. The computer programmedium may be, for example but not limited to, an electric, magnetic,optical, infrared or semiconductor system, device or transmissionmedium. The computer program medium may include at least one of thefollowing media: a computer readable medium, a program storage medium, arecord medium, a computer readable memory, a random access memory, anerasable programmable read-only memory, a computer readable softwaredistribution package, a computer readable signal, a computer readabletelecommunications signal, computer readable printed matter, and acomputer readable compressed software package.

Even though the invention has been described above with reference to anexample according to the accompanying drawings, it is clear that theinvention is not restricted thereto but it can be modified in severalways within the scope of the appended claims.

1. A method of forming an antenna beam pattern in a radio system, themethod comprising: forming a fixed phase shift for each of a pluralityof radio frequency signal components directed to or received from theplurality of antenna elements; and forming a desired antenna beampattern with at least one grating lobe on the basis of the phase-shiftedradio frequency signal components of the antenna elements in apredefined antenna structure for deliberately utilizing the at least onegrating lobe in a transmission of data between a transmitter and areceiver.
 2. The method of claim 1, further comprising forming a desiredantenna beam pattern with an antenna having a spacing d between antennaelements larger than λ/(1+sin|θ_(max)|) where λ is a wavelength of acarrier and θ_(max) is the maximum scan angle.
 3. The method of claim 1,further comprising: dividing a radio frequency signal into signalcomponents directed to a plurality of antenna elements; and forming adesired transmission antenna beam pattern on the basis of thephase-shifted radio frequency signal components directed to thedifferent antenna elements.
 4. The method of claim 1, furthercomprising: receiving radio frequency signal components from a pluralityof antenna elements; and forming a desired reception antenna beampattern on the basis of the phase-shifted radio frequency signalcomponents received from the plurality of antenna elements.
 5. Themethod of claim 1, further comprising forming the fixed phase shift foreach radio frequency signal using a Butler matrix or a correspondinganalog beam forming component.
 6. The method of claim 1, furthercomprising forming an antenna beam pattern with different beams fordifferent receivers.
 7. The method of claim 1, by further comprisingforming an antenna beam pattern with different beams for different data,the data being for a receiver.
 8. The method of claim 1, by furthercomprising: encoding data; dividing the encoded data into at least twoparts; forming an antenna beam pattern with different beams; andtransmitting the parts of the encoded data in different beams to areceiver.
 9. The method of claim 1, further comprising forming, atdifferent moments, antenna beam patterns of different beams fordifferent receivers, different beams for different data, and/ordifferent beams with different parts of encoded data.
 10. The method ofclaim 1, further comprising receiving encoded data from at least twobeams and decoding the data.
 11. An antenna, comprising: a phaseshifter; and a plurality of elements the phase shifter being configuredto form a fixed phase shift for each of a plurality of radio frequencysignal components directed to or received from the plurality of antennaelements, the antenna elements having a structure configured to form adesired antenna beam pattern with at least one grating lobe using thefixed phase shifts on the basis of the phase-shifted radio frequencysignal components of the antenna elements for deliberately utilizing theat least one grating lobe in a transmission of data between atransmitter and a receiver.
 12. The antenna of claim 11, wherein theantenna elements have a spacing d between each other larger thanλ/(1+sin|θ_(max)|) where λ is a wavelength of a carrier and θ_(max) isthe maximum scan angle.
 13. The antenna of claim 11, wherein the phaseshifter is configured to divide a radio frequency signal into aplurality of antenna elements, the antenna elements having a structureconfigured to form a desired transmission antenna beam pattern with thefixed phase shifts.
 14. The antenna of claim 11, wherein the phaseshifter and the antenna elements are integrated together.
 15. Theantenna of claim 11, wherein the phase shifter is a Butler matrix or acorresponding analog beam forming component.
 16. A transmitter, thetransmitter comprising an antenna which comprises: a phase shifter and aplurality of elements, the phase shifter being configured to form afixed phase shift for each of a plurality of radio frequency signalcomponents directed to the plurality of antenna elements, the antennaelements having an antenna structure configured to form a desiredantenna beam pattern with at least one grating lobe using the fixedphase shifts on the basis of the phase-shifted radio frequency signalcomponents of the antenna elements for deliberately utilizing the atleast one grating lobe in a transmission of data from the transmitter.17. The transmitter of claim 16, wherein the transmitter is configuredto form an antenna beam pattern with different beams for differentreceivers.
 18. The transmitter of claim 16, wherein the transmitter isconfigured to form an antenna beam pattern with different beams fordifferent data.
 19. The transmitter of claim 16, wherein the transmitteris configured to encode data, to divide the encoded data into at leasttwo parts, to form an antenna beam pattern with different beams and totransmit the parts of the encoded data in different beams to a receiver.20. The transmitter of claim 16, wherein the transmitter is configuredto form, at different moments, antenna beam patterns of different beamsfor different receivers, different beams for different data, and/ordifferent beams with different parts of encoded data.
 21. A receiver,the receiver comprising an antenna which comprises: a phase shifter; anda plurality of elements, the phase shifter being configured to form afixed phase shift for each of a plurality of radio frequency signalcomponents received from the plurality of antenna elements, the antennaelements having an antenna structure configured to form a desiredantenna beam pattern with at least one grating lobe using the fixedphase shifts on the basis of the phase-shifted radio frequency signalcomponents of different antenna elements for deliberately utilizing theat least one grating lobe in a reception of data in a receiver.
 22. Thereceiver of claim 21, wherein the phase shifter is configured to combinethe received radio frequency signals from the antenna elements, theantenna elements having a structure configured to form a desiredreception antenna beam pattern with the fixed phase shifts.
 23. Thereceiver of claim 21, wherein the receiver is configured to receiveencoded data from at least two beams and to decode the data.